Fire an accessibility change event in Button::SetAccessibleName.

This allows a consumer to call SetAccessibleName multiple times and
have the new name be reflected in ChromeVox.

Bug: 719015
Change-Id: If355c3988907f67f336255a9b146e742e44bf9e2
Reviewed-on: https://chromium-review.googlesource.com/709921Reviewed-by: Michael Wasserman <msw@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: Jacob Dufault <jdufault@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521172}

ui/views/controls/button/button.cc

@@ -106,6 +106,7 @@ void Button::SetTooltipText(const base::string16& tooltip_text) {
 
 void Button::SetAccessibleName(const base::string16& name) {
   accessible_name_ = name;
+  NotifyAccessibilityEvent(ui::AX_EVENT_TEXT_CHANGED, true);
 }
 
 void Button::SetState(ButtonState state) {